Past exam of the year 2 of the earth sciences (geology) course of the University of Oxford / 2022 / Paper 1 / 4 / d / iii / Solution

If a restored channel or fan boundary has accumulated displacement $d$ since it formed, a constant fault slip rate $v=1.6\mathrm{mm}\mathrm{y}{\mathrm{r}}^{-1}$ gives the geomorphic offset-age relationThe right-hand channel jog is of order one to two of the printed $100$ m scale bars, so apparent offsets of order $100$–$200$ m give about $62.5$–$125$ ka. These are scale-based order-of-magnitude estimates, not surveyed piercing-point offsets: the amount of deflection along an eroding channel is not automatically equal to the tectonic displacement. For example, a reliably restored $150$ m marker would imply $93.75$ ka.

These times lie in the late Pleistocene, when global glacial–interglacial changes could alter vegetation, sediment supply and runoff and thereby promote alluvial fan deposition or abandonment. The wide interval spans different climatic phases, so it does not identify one event or prove a climatic cause. More importantly, a channel cut into a fan usually dates channel establishment or abandonment; the fan sediment can be older. Independent optically stimulated luminescence dating, stratigraphic correlation and several matched offsets are needed to assign a depositional age and test a climatic correlation.